Rab's Module is ideal for ultra lightweight use, mountain marathons, bivies, and adventure racing.
Here's what other sites are saying:
The Rab Module Liner is an ultra-lightweight sleeping bag for light and fast use, it has down on the top only and must be used with a sleeping Pad for protection under that bag where there is no bottom. As this is technically only 'half' of a sleeping bag it is not possible to test it using EN13537 testing procedure - as such no official temperature limits can be given. The Module is designed primarily as a 'booster' bag o be used inside an existing bag to add extra warmth and comfort. The Module also doubles as a very lightweight sleeping bag. Ideal for mountain marathon, adventure racing, as aliner bag for an ultra-lightweight expedition.